Move s3 settings to configuration

[tarunbhardwaj]

The current design has several issues:

• stores credentials in database (while most users are careful, a leak of the credentials could mean damage if the IAM key has more powers than it deserves)
• The backend type selection at file level is confusing. It should be at folder level.
• The public and private nature largely depends on how the bucket is configured.

Tasks

• Add s3 as a type to folder.
• If type is s3, then ask if the content should be private is_private
• Ensure that the folder name cannot be _private since that would be the prefix for private files
• Ensure that the is_private flag cannot be changed once files are added to a folder.

Documentation:

• Steps for migrating from previous version
• Write an example bucket policy and credential policy with minimal permissions.